Quick questions on DNA structure: Watson, Crick, Franklin and Wilkins (HSC Biology Module 5)

The two strands run in opposite directions. One runs 5' to 3'; the other runs 3' to 5'. This antiparallel orientation matters for DNA replication (DNA polymerase only synthesises in the 5' to 3' direction, which produces the leading vs lagging strand distinction).

The two strands twist around each other in a right-handed double helix. One full turn is roughly 3.4 nm and contains about 10 base pairs. The diameter is about 2 nm.

A skilled X-ray crystallographer. In 1952 she produced Photograph 51, an X-ray diffraction image of DNA that clearly showed the helical structure and the regular spacing of the bases. She was on the verge of publishing her own model.

Franklin's colleague. Showed Photograph 51 to Watson without Franklin's knowledge or permission. Wilkins shared the 1962 Nobel Prize with Watson and Crick; Franklin had died in 1958 and was not eligible.

Built the first accurate physical model of the DNA double helix in 1953, using Franklin's X-ray data plus Erwin Chargaff's chemical analysis showing $A = T$ and $G = C$. Their paper "Molecular Structure of Nucleic Acids" was published in _Nature_ on 25 April 1953. It is one of the most cited papers in biology.

Showed in the 1940s that in any DNA sample, the amount of A equals the amount of T, and G equals C. These Chargaff's rules were the crucial constraint Watson and Crick used to figure out base pairing.

A-T, G-C. Not A-G or T-C. A and G are purines (large); T and C are pyrimidines (small).

A-T has 2 bonds; G-C has 3 bonds. This is why G-C-rich DNA is harder to separate (more energy needed to break more hydrogen bonds).

A double-stranded DNA sample contains 18 percent guanine. Calculate the percentage of adenine, thymine and cytosine in the sample. [3 marks]

Justify the inclusion of Rosalind Franklin in any historical account of the DNA double helix. [3 marks]

A bacterial DNA sample melts (the two strands separate) at 95 degrees C. A second sample from a different organism melts at 88 degrees C. (a) Account for the difference in melting temperatures.